Gallium nitride (GaN)-based thin film crystal draws attention as excellent blue light-emitting devices, has been used as a material for light-emitting diodes and expected as a blue-violet semiconductor laser device for an optical pickup. In recent years, the GaN thin film crystal draws attention as a semiconductor film composing an electronic device, such as a high-speed IC chip for use in cellular phones.
There is reported a method for depositing a seed crystal film of GaN or AlN on a single crystal substrate composed of sapphire to obtain a template substrate, and growing a GaN single crystal on the template substrate (see Japanese Patent Laid-open Publication No. 2000-327495A).
According to Formation chemistry of high-density nanocraters on the surface of sapphire substrates with an in-situ etching and growth mechanism of device-quality GaN films on the etched substrates” (“Applied Physics Letters”, Volume 84, Number 20, pp. 4041 to 4043), after a GaN thin film is grown on a sapphire substrate, the GaN thin film and the sapphire substrate surface are in-situ etched by hydrogen annealing. This process makes it possible not only to almost eliminate the GaN thin film, but also to generate a number of minute microcraters on the sapphire substrate surface and to roughen the surface. When the GaN single crystal film is grown again on the sapphire substrate surface, voids are formed thereon. It is reported that the crystallinity of the GaN single crystal film can be remarkably improved according to the process.
Japanese Patent Laid-open Publication No. 2004-247711A discloses the following method. Voids are formed within a seed crystal substrate by ELO (Epitaxial Lateral Overgrowth) technique, and a c-plane GaN is grown over the substrate using melt containing an alkaline metal as represented by Na flux. As a result, the c-plane GaN and the sapphire substrate are separated from each other at the void region as their border, thereby obtaining a GaN single crystal self-standing substrate.